Semiconductor device and method for manufacturing the same

ABSTRACT

A first conductive film overlapping with an oxide semiconductor film is formed over a gate insulating film, a gate electrode is formed by selectively etching the first conductive film using a resist subjected to electron beam exposure, a first insulating film is formed over the gate insulating film and the gate electrode, removing a part of the first insulating film while the gate electrode is not exposed, an anti-reflective film is formed over the first insulating film, the anti-reflective film, the first insulating film and the gate insulating film are selectively etched using a resist subjected to electron beam exposure, and a source electrode in contact with one end of the oxide semiconductor film and one end of the first insulating film and a drain electrode in contact with the other end of the oxide semiconductor film and the other end of the first insulating film are formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device generally refers to adevice which can function by utilizing semiconductor characteristics,and an electrooptic device, a semiconductor circuit, and electronicequipment are all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a thin film transistor (TFT)). Thetransistor is applied to a wide range of electronic devices such as anintegrated circuit (IC) or an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a transistor whose active layer includes an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) is disclosed (seePatent Document 1).

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2006-165528 SUMMARY OF THE INVENTION

In order to achieve high-speed operation, low power consumption, highintegration, or the like of a transistor, it is necessary to miniaturizea transistor.

One object of an embodiment of the present invention is to provide astructure of a semiconductor device which achieves high-speed responseand high-speed operation by improving on-state characteristics of aminiaturized transistor (e.g., on-state current or field-effectmobility), and to provide a manufacturing method thereof, in order toachieve a high-performance semiconductor device.

Further, in accordance with miniaturization of the transistor, concernabout a decrease in yield of a manufacturing process rises.

Accordingly, it is another object to provide a minute transistor havinghigh electric characteristics with high yield.

Further, another object of an embodiment of the present invention is toachieve high performance, high reliability, and high productivity alsoin the semiconductor device including the transistor.

In a semiconductor device including a transistor in which an oxidesemiconductor film, a gate insulating film, and a gate electrode on sidesurfaces and a top surface of which a first insulating film is providedare stacked in this order, a source electrode and a drain electrode areprovided in contact with the oxide semiconductor film and the firstinsulating film. In a process for manufacturing the semiconductordevice, a conductive film and a second insulating film are stacked tocover the oxide semiconductor film, the first insulating film, and thegate electrode. Then, the second insulating film and the conductive filmare removed (or polished), so that the conductive film over the gateelectrode is removed. Accordingly, a source electrode and a drainelectrode are formed. As the removing (or polishing) method, a chemicalmechanical polishing (CMP) method can be preferably used.

Further, an insulating film (a sidewall insulating film) on a sidesurface of a gate electrode is formed using a resist mask, and theheight of a gate electrode region becomes high by using the insulatingfilm at the same time. Here, the “height of a gate electrode region” inthis specification refers to the height from a bottom surface of a gateelectrode to a top surface of a film which is in contact with the gateelectrode. The height of the gate electrode region is high, whereby thesource electrode and the drain electrode can be easily separated fromeach other.

Further, when a resist mask having a miniaturized line formed byexposing a resist to an electron beam is used, a channel length can beshortened. In consideration of etching selectivity of films to eachother which are to be deposited, specifically, a hard mask film isprovided over a conductive film, a resist formed on the hard mask filmis exposed to an electron beam, the hard mask film is etched using thedeveloped resist mask as an etching mask, the conductive film is etchedusing the etched hard mask film as a mask, so that a gate electrode isformed. A region overlapping with the gate electrode in the oxidesemiconductor film becomes a channel formation region of a transistor.

One embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film over an insulating surface, a gateinsulating film over the oxide semiconductor film, a gate electrodeprovided over the gate insulating film and overlapping with the oxidesemiconductor film, a first insulating film over the gate insulatingfilm and the gate electrode, a source electrode in contact with one endof the oxide semiconductor film and one end of the first insulatingfilm, a drain electrode in contact with the other end of the oxidesemiconductor film and the other end of the first insulating film, and asecond insulating film over the source electrode and the drainelectrode. Heights of top surfaces of the source electrode and the drainelectrode are substantially the same as heights of top surfaces of thefirst insulating film and the second insulating film. A channel lengthof the oxide semiconductor film is greater than or equal to 1 nm andless than or equal to 30 nm.

Another embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film provided over an insulatingsurface and including a channel formation region, and a firstlow-resistance region and a second low-resistance region with thechannel formation region sandwiched therebetween, a gate insulating filmover the oxide semiconductor film, a gate electrode provided over thegate insulating film and overlapping with the channel formation region,a first insulating film over the gate insulating film and the gateelectrode, a source electrode in contact with a part of the firstlow-resistance region, a drain electrode in contact with a part of thesecond low-resistance region, and a second insulating film over thesource electrode and the drain electrode. Heights of top surfaces of thesource electrode and the drain electrode are substantially the same asheights of top surfaces of the first insulating film and the secondinsulating film. A channel length of the oxide semiconductor film isgreater than or equal to 1 nm and less than or equal to 30 nm

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of forming anoxide semiconductor film over an insulating surface, forming a gateinsulating film covering the oxide semiconductor film, forming a firstconductive film provided over the gate insulating film and overlappingwith the oxide semiconductor film, forming a hard mask film over thefirst conductive film, forming a first resist by performing electronbeam exposure over the hard mask film, selectively etching the hard maskfilm, forming a gate electrode by selectively etching the firstconductive film using the etched hard mask film as a mask, forming afirst insulating film over the gate insulating film and the gateelectrode, performing removing treatment on a part of the firstinsulating film while the gate electrode is not exposed, forming ananti-reflective film over the first insulating film subjected to theremoving treatment, forming a second resist by performing electron beamexposure provided over the anti-reflective film and overlapping with theoxide semiconductor film, exposing parts of the insulating surface andthe oxide semiconductor film by selectively etching the anti-reflectivefilm, the first insulating film, and the gate insulating film, forming asecond conductive film over the exposed insulating surface, oxidesemiconductor film, and anti-reflective film, forming a secondinsulating film over the second conductive film, performing removingtreatment on parts of the second insulating film and the secondconductive film, and the anti-reflective film so that the firstinsulating film is exposed, and forming a source electrode and a drainelectrode by processing the second conductive film subjected to theremoving treatment.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of forming anoxide semiconductor film over an insulating surface, forming a gateinsulating film covering the oxide semiconductor film, forming a firstconductive film provided over the gate insulating film and overlappingwith the oxide semiconductor film, forming a hard mask film over thefirst conductive film, forming a first resist by performing electronbeam exposure over the hard mask film, selectively etching the hard maskfilm, forming a gate electrode by selectively etching the firstconductive film using the etched hard mask film as a mask, forming, in aself-aligned manner, a channel formation region in a region overlappingwith the gate electrode in the oxide semiconductor film and a firstlow-resistance region and a second low-resistance region between whichthe channel formation region is sandwiched in the oxide semiconductorfilm by adding impurities, forming a first insulating film over the gateinsulating film and the gate electrode, performing removing treatment ona part of the first insulating film while the gate electrode is notexposed, forming an anti-reflective film over the first insulating filmon which the removing treatment is performed, forming a second resistprovided by performing electron beam exposure over the anti-reflectivefilm and overlapping with the channel formation region, the firstlow-resistance region, and the second low-resistance region, exposingparts of the insulating surface, the first low-resistance region, andthe second low-resistance region by selectively etching theanti-reflective film, the first insulating film, and the gate insulatingfilm, forming a second conductive film over the exposed insulatingsurface, first low-resistance region, and second low-resistance region,and the anti-reflective film, forming a second insulating film over thesecond conductive film, performing removing treatment on parts of thesecond insulating film and the second conductive film, and theanti-reflective film so that the first insulating film is exposed, andforming a source electrode and a drain electrode by processing thesecond conductive film on which the removing treatment is performed.

According to one embodiment of the present invention, in the abovemanufacturing method, the removing treatment is preferably performed bychemical mechanical polishing.

According to one embodiment of the present invention, in the abovemanufacturing method, the hard mask film is preferably a stacked film ofa silicon nitride oxide film and an amorphous silicon film or a stackedfilm of a silicon oxide film and an amorphous silicon film.

According to one embodiment of the present invention, in the abovemanufacturing method, a second hard mask film is formed after theanti-reflective film is formed and before the second resist is formed.The second hard mask film is preferably a stacked film of a siliconnitride oxide film and an amorphous silicon film or a stacked film of asilicon oxide film and an amorphous silicon film.

According to one embodiment of the present invention, in the abovemanufacturing method, a channel length of the oxide semiconductor filmis determined by electron beam exposure.

The oxide semiconductor film is preferably highly purified so as tocontain hardly any impurities such as copper, aluminum, and chlorine. Inthe process for manufacturing the transistor, steps in which theseimpurities are not mixed or attached to the surface of the oxidesemiconductor film are preferably selected as appropriate. In the casewhere the impurities are attached to the surface of the oxidesemiconductor film, the impurities on the surface of the oxidesemiconductor film are preferably removed by exposure to oxalic acid,dilute hydrofluoric acid, or the like or by plasma treatment (such asN₂O plasma treatment). Specifically, the copper concentration of theoxide semiconductor film is lower than or equal to 1×10¹⁸ atoms/cm³,preferably lower than or equal to 1×10¹⁷ atoms/cm³. In addition, thealuminum concentration of the oxide semiconductor film is lower than orequal to 1×10¹⁸ atoms/cm³. Further, the chlorine concentration of theoxide semiconductor film is lower than or equal to 2×10¹⁸ atoms/cm³.

In addition, shortly after the oxide semiconductor film is formed, it ispreferable that the oxide semiconductor film contains oxygen in aproportion higher than that in the stoichiometric composition, i.e., theoxide semiconductor film is supersaturated. For example, in the casewhere the oxide semiconductor film is formed by a sputtering method, theformation is preferably performed in the state where the proportion ofoxygen in a film formation gas is large, and in particular, theformation is preferably performed in an oxygen atmosphere (an oxygengas: 100%). When the formation is performed in the state where theproportion of oxygen in the film formation gas is large, particularly ina 100% oxygen gas atmosphere, release of Zn from the film can be reducedeven at a film formation temperature higher than or equal to 300° C.,for example.

The oxide semiconductor film is preferably an oxide semiconductor filmwhich is purified by sufficient removal of impurities such as hydrogenor by sufficient supply of oxygen so as to be supersaturated withoxygen. Specifically, the hydrogen concentration of the oxidesemiconductor film is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less. Note thatthe above hydrogen concentration of the oxide semiconductor film ismeasured by secondary ion mass spectrometry (SIMS). In order that theoxide semiconductor film is supersaturated with oxygen by sufficientsupply of oxygen, an insulating film containing excess oxygen (such as aSiO_(x) film) is provided so as to cover and be in contact with theoxide semiconductor film.

As the insulating film containing excess oxygen, a SiO_(x) or siliconoxynitride film containing much oxygen as a result of film formationunder the conditions which are set as appropriate for a plasma CVDmethod or a sputtering method is used. In order to make the insulatingfilm contain much more excess oxygen, oxygen is added by an ionimplantation method, an ion doping method, or plasma treatment.

In the case where the hydrogen concentration of the insulating filmcontaining excess oxygen is greater than or equal to 7.2×10²⁰ atoms/cm³,variation in initial characteristics of transistors is increased, achannel length dependence is increased, and a transistor issignificantly deteriorated in the BT stress test; therefore, thehydrogen concentration of the insulating film containing excess oxygenshould be less than 7.2×10²⁰ atoms/cm³. In other words, the hydrogenconcentration of the oxide semiconductor film is preferably less than orequal to 5×10¹⁹ atoms/cm³, and the hydrogen concentration of theinsulating film containing excess oxygen is preferably less than7.2×10²⁰ atoms/cm³.

In addition, a blocking film (such as an AlO_(x) film) for preventingoxygen from being released from the oxide semiconductor film ispreferably provided so as to be positioned outside the insulating filmcontaining excess oxygen.

When the oxide semiconductor film is surrounded by the insulating filmcontaining excess oxygen or the blocking film, the oxide semiconductorfilm can contain oxygen in a proportion higher than that in thestoichiometric composition i.e., the oxide semiconductor film can besupersaturated with oxygen. For example, in the case where thestoichiometric composition of the oxide semiconductor film isIn:Ga:Zn:O=1:1:1:4 [atomic ratio], the ratio of oxygen atoms in the IGZOis larger than 4.

Accordingly, a minute transistor having high electric characteristicscan be provided with high yield.

Moreover, a minute transistor having a short channel length can beachieved, whereby the operation speed of a circuit can be increased andpower consumption can be reduced.

Further, also in a semiconductor device including the transistor, highperformance, high reliability, and high productivity can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a plan view and a cross-sectional view illustratinga semiconductor device according to one embodiment of the presentinvention;

FIGS. 2A to 2D are cross-sectional views illustrating a manufacturingprocess of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 3A to 3D are cross-sectional views illustrating the manufacturingprocess of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 4A to 4D are cross-sectional views illustrating the manufacturingprocess of the semiconductor device according to one embodiment of thepresent invention;

FIGS. 5A and 5B are a plan view and a cross-sectional view illustratinga semiconductor device according to one embodiment of the presentinvention;

FIGS. 6A to 6C are a cross-sectional view, a plan view, and a circuitdiagram illustrating one embodiment of a semiconductor device;

FIGS. 7A and 7B are a circuit diagram and a perspective viewillustrating one embodiment of a semiconductor device;

FIGS. 8A and 8B are a cross-sectional view and a plan view illustratingthe one embodiment of the semiconductor device;

FIGS. 9A and 9B are circuit diagrams each illustrating one embodiment ofa semiconductor device;

FIG. 10 is a block diagram illustrating one embodiment of asemiconductor device;

FIG. 11 is a block diagram illustrating one embodiment of asemiconductor device; and

FIG. 12 is a block diagram illustrating one embodiment of asemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that a variety of changesand modifications can be made without departing from the spirit andscope of the present invention. Accordingly, the invention should not beconstrued as being limited to the description of the embodiments below.In describing structures of the present invention with reference to thedrawings, the same reference numerals are used in common for the sameportions in different drawings. The same hatching pattern is applied tosimilar parts, and the similar parts are not especially denoted byreference numerals in some cases. In addition, for convenience, aninsulating film is not illustrated in a top view in some cases.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating film” can mean the case wherethere is an additional component between the gate insulating film andthe gate electrode.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit a function of a component. Forexample, an “electrode” is sometimes used as part of a “wiring”, andvice versa. In addition, the term “electrode” or “wiring” can also meana combination of a plurality of “electrodes” and “wirings”, for example.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be replaced witheach other in this specification.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object.

Examples of an “object having any electric function” are an electrodeand a wiring.

Furthermore, hereinafter, ordinal numbers, such as “first” and “second,”are used merely for convenience, and the present invention is notlimited to the numbers.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device, which is oneembodiment of the present invention, will be described with reference toFIGS. 1A and 1B, FIGS. 2A to 2D, FIGS. 3A to 3D, and FIGS. 4A to 4D.

FIGS. 1A and 1B are a plan view and a cross-sectional view of atransistor 450. FIG. 1A is a plan view and FIG. 1B is a cross-sectionalview taken along line A-B in FIG. 1A. Note that in FIG. 1A, somecomponents (e.g., a base insulating film 432) of the transistor 450 areomitted to avoid complexity.

<Structure of Semiconductor Device According to this Embodiment>

FIGS. 1A and 1B are an example of a structure of a semiconductor devicemanufactured according to a method of this embodiment. The transistor450 illustrated in FIGS. 1A and 1B includes a base insulating film 432provided over a substrate 400 having an insulating surface, an oxidesemiconductor film 403 including a channel formation region 403 c andlow-resistance regions 403 a and 403 b with the channel formation region403 c sandwiched therebetween over the base insulating film 432, a gateinsulating film 412 a over the oxide semiconductor film 403, a gateelectrode 401 a provided over the gate insulating film 412 a andoverlapping with the channel formation region 403 c, an insulating film415 b over the gate insulating film 412 a and the gate electrode 401 a,a source electrode 405 a overlapping with parts of the base insulatingfilm 432 and the low-resistance region 403 a, a drain electrode 405 boverlapping with parts of the base insulating film 432 and thelow-resistance region 403 b, and an insulating film 425 a over thesource electrode 405 a and the drain electrode 405 b.

The insulating film 415 b is provided over the gate electrode 401 a,whereby the height of the gate electrode region becomes high and asidewall insulating film can be formed on a side surface of a gateelectrode 401 a. Thus, when removing (polishing) treatment is performedon a part of a conductive film to be a source electrode and a drainelectrode, the source electrode 405 a and the drain electrode 405 b canbe easily separated from each other.

Further, the length of the gate electrode 401 a in the channel lengthdirection can be determined by exposure to an electron beam. Here, aportion in the oxide semiconductor film 403 overlapping with a regionwhere the gate electrode 401 a is formed becomes a channel formationregion of the transistor. That is, since the channel length can bedetermined by the exposure to an electron beam, a transistor with asmall channel length can be manufactured.

Further, the source electrode 405 a and the drain electrode 405 b areprovided in contact with the exposed portion of a top surface of theoxide semiconductor film 403 and the insulating film 415 b. Therefore,the distance L1 between the gate electrode 401 a and the region (contactregion) in which the oxide semiconductor film 403 is in contact with thesource electrode 405 a (or the drain electrode 405 b) can be determinedby the exposure to an electron beam, so that the resistance between thegate electrode 401 a and the region (contact region) in which the oxidesemiconductor film 403 is in contact with the source electrode 405 a (orthe drain electrode 405 b) is reduced; thus, the on-statecharacteristics of the transistor 450 can be improved.

<Manufacturing Method of Semiconductor Device According to thisEmbodiment>

A method for manufacturing the transistor 450 will be described withreference to FIGS. 2A to 2D, FIGS. 3A to 3D, and FIGS. 4A and 4D.

First, the base insulating film 432 is formed over the substrate 400 andthe oxide semiconductor film 403 is formed over the base insulating film432 (see FIG. 2A).

For the substrate 400, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used. Asingle crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like; acompound semiconductor substrate made of silicon germanium or the like;an SOI (silicon on insulators) substrate; or the like can be used as thesubstrate 400, or the substrate provided with a semiconductor elementcan be used as the substrate 400.

The base insulating film 432 is formed by a plasma CVD method or asputtering method to have a thickness greater than or equal to 50 nm andless than or equal to 2 μm with the use of one of a silicon oxide film,a gallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, and a siliconnitride oxide film or a stack of any of these films. The base insulatingfilm 432 can prevent entry of impurities from the substrate 400 side. Inthe case where the base insulating film 432 is unnecessary, e.g., in thecase where the amount of moisture adsorbed on a surface of the substrate400 and the amount of moisture included in the substrate 400 are small,the base insulating film 432 is not necessarily provided.

It is preferable that an insulating film from which oxygen is releasedby heat treatment be used as the base insulating film 432.

Note that “oxygen is released by heat treatment” described above refersto an amount of released oxygen when converted into oxygen atoms inthermal desorption spectroscopy (TDS) analysis is greater than or equalto 1.0×10¹⁹ atoms/cm³, preferably greater than or equal to 3.0×10¹⁹atoms/cm³, further preferably greater than or equal to 1.0×10²⁰atoms/cm³, still further preferably greater than or equal to 3.0×10²⁰atoms/cm³.

Here, a method in which the amount of released oxygen is measured bybeing converted into oxygen atoms using the TDS analysis is describedbelow.

The released amount of gas in the TDS analysis is proportional to anintegral value of spectrum. Therefore, the amount of released gas can becalculated from the ratio between the integral value of a measuredspectrum and the reference value of a standard sample. The referencevalue of a standard sample refers to the ratio of the density of apredetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of released oxygen molecules (N_(O2)) from aninsulating film can be found according to Formula (I) with the TDSanalysis results of a silicon wafer containing hydrogen at apredetermined density which is the standard sample and the TDS analysisresults of the insulating film. Here, all spectra having amass-to-charge ratio (M/z) of 32 which are obtained by the TDS analysisare assumed to originate from an oxygen molecule. CH₃OH, which is givenas a gas where M/z=32, is not taken into consideration on the assumptionthat it is unlikely to be present. Further, an oxygen molecule includingan oxygen atom where M/z=17 or 18 which is an isotope of an oxygen atomis not taken into consideration either because the proportion of such amolecule in the natural world is minimal.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 1} \rbrack \mspace{574mu}} & \; \\{N_{O\; 2} = {\frac{N_{H\; 2}}{S_{H\; 2}} \times S_{O\; 2} \times \alpha}} & (1)\end{matrix}$

N_(H2) is the value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into densities. S_(H2) isthe integral value of a spectrum when the standard sample is subjectedto the TDS analysis. Here, the reference value of the standard sample isset to N_(H2)/S_(H2). S_(O2) is the integral value of a spectrum whenthe insulating film is subjected to the TDS analysis. α is a coefficientaffecting the intensity of the spectrum in the TDS analysis. Refer toJapanese Published Patent Application No. H06-275697 for details of theFormula I. Note that the amount of released oxygen from the aboveinsulating film is measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon wafercontaining hydrogen atoms at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above α is determined with considering the ionization rate ofthe oxygen molecules, the number of the released oxygen atoms can alsobe estimated through the evaluation of the number of the released oxygenmolecules.

Note that N_(O2) is the number of the released oxygen molecules. Theamount of released oxygen when converted into oxygen atoms is twice thenumber of the released oxygen molecules.

Note that in this specification, “oxynitride” such as silicon oxynitridecontains more oxygen than nitrogen.

Further, in this specification, “nitride oxide” such as silicon nitrideoxide contains more nitrogen than oxygen.

Further, an aluminum oxide film is preferably provided between thesubstrate 400 and the base insulating film 432. In particular, it ispreferable to use an aluminum oxide film with a film density of higherthan or equal to 3.2 g/cm³, preferably higher than or equal to 3.6g/cm³. The aluminum oxide film has a thickness greater than or equal to30 nm and less than or equal to 150 nm, preferably greater than or equalto 50 nm and less than or equal to 100 nm. When the film density of thealuminum oxide film is within the above range, moisture or hydrogen canbe prevented from entering and diffusing into the oxide semiconductorfilm. In addition, release of oxygen from the oxide semiconductor film403 can be suppressed.

Hydrogen or water is preferably removed from the base insulating film432 by heat treatment at a temperature lower than or equal to 650° C.

The oxide semiconductor film 403 can be formed by a sputtering method, amolecular beam epitaxy (MBE) method, a CVD method, a pulsed laserdeposition method, an atomic layer deposition (ALD) method, or the like.The oxide semiconductor film 403 may be formed with the use of asputtering apparatus which performs deposition in the state where topsurfaces of a plurality of substrates are substantially perpendicular toa top surface of a sputtering target. In this embodiment, etchingtreatment is performed on the oxide semiconductor film 403 and the filmis formed in an island shape; however, the prevent invention is notlimited thereto.

In the formation of the oxide semiconductor film 403, the concentrationof hydrogen contained in the oxide semiconductor film 403 is preferablyreduced. In order to reduce the concentration of hydrogen contained inthe oxide semiconductor film 403, for example, in the case where theoxide semiconductor film is formed by a sputtering method, a high-purityoxygen, a rare gas (typically, argon), or a mixed gas of oxygen and therare gas from which impurities such as hydrogen, water, a hydroxylgroup, or hydride have been removed is used as appropriate as anatmosphere gas supplied to a deposition chamber of a sputteringapparatus.

The oxide semiconductor film 403 is formed in such a manner that asputtering gas from which hydrogen and moisture have been removed isintroduced into a deposition chamber while moisture remaining in thedeposition chamber is removed, whereby the concentration of hydrogen inthe formed oxide semiconductor film 403 can be reduced. In order toremove moisture remaining in the deposition chamber, an entrapmentvacuum pump such as a cryopump, an ion pump, or a titanium sublimationpump is preferably used. The evacuation unit may be a turbo molecularpump provided with a cold trap. A cryopump has a high capability inremoving a compound containing a hydrogen atom, such as a hydrogenmolecule and water (H₂O) (preferably, also a compound containing acarbon atom), and the like; therefore, the concentration of impuritiescontained in the oxide semiconductor film 403 formed in the depositionchamber which is evacuated with a cryopump can be reduced.

Further, when the oxide semiconductor film 403 is formed by a sputteringmethod, the relative density (fill rate) of a metal oxide target that isused for forming the oxide semiconductor film 403 is greater than orequal to 90% and less than or equal to 100%, preferably greater than orequal to 95% and less than or equal to 99.9%. With the use of a metaloxide target with a high relative density, the formed oxidesemiconductor film 403 can be dense.

As a material of the oxide semiconductor film 403, for example, anIn-M-Zn—O-based material may be used. Here, a metal element M is anelement whose bond energy with oxygen is higher than that of In and thatof Zn. Alternatively, M is an element which has a function ofsuppressing desorption of oxygen from the In-M-Zn—O-based material.Owing to the effect of the metal element M, generation of oxygenvacancies in the oxide semiconductor film is suppressed. Therefore,change in electrical characteristics of the transistor, which is causedby oxygen vacancies, can be reduced; accordingly, a highly reliabletransistor can be obtained.

The metal element M may be, specifically, Al, Sc, Ti, V, Cr, Mn, Fe, Co,Ni, Ga, Y, Zr, Nb, Mo, Sn, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Hf, Ta, or W, and is preferably Al, Ti, Ga, Y, Zr, Ce, orHf. The metal element M may be formed using one or more elementsselected from the above elements. Further, Si or Ge can be used insteadof the metal element M.

Here, in the In-M-Zn—O-based material which is an oxide semiconductor,the higher the concentration of In is, the higher the carrier mobilityand the carrier density are. As a result, the oxide semiconductor hashigher conductivity as the concentration of In is higher.

The oxide semiconductor film 403 may have either a single-layerstructure or a stacked structure. The oxide semiconductor film 403 maybe in a single crystal state, a polycrystalline (also referred to aspolycrystal) state, or an amorphous state.

In this embodiment, the oxide semiconductor film 403 is preferably ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not clear. Further, with the TEM, a grainboundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, areduction in electron mobility, due to the grain boundary, issuppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

For the formation of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe film formation, the crystal state can be prevented from being brokenby the impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the filmformation, migration of a sputtered particle is likely to occur afterthe sputtered particle reaches a substrate surface. Specifically, thesubstrate heating temperature during the film formation is higher thanor equal to 100° C. and lower than or equal to 740° C., preferablyhigher than or equal to 200° C. and lower than or equal to 500° C. Byincreasing the substrate heating temperature during the film formation,when the flat-plate-like sputtered particle reaches the substrate,migration occurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the film formation. The proportion of oxygen inthe deposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn-based oxide targetis described below.

The In—Ga—Zn-based oxide target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined molar ratio of InO_(X) powderto GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratiofor mixing powder may be determined as appropriate depending on thedesired sputtering target.

The oxide semiconductor film is preferably in a supersaturated state inwhich oxygen which exceeds the stoichiometric composition is containedjust after its formation. For example, when an oxide semiconductor filmis formed by a sputtering method, it is preferable that the film beformed in a film formation gas containing a high percentage of oxygen,and it is especially preferable that the film be formed under an oxygenatmosphere (oxygen gas 100%). When deposition is performed under such acondition that the ratio of oxygen to a deposition gas is high,particularly in an atmosphere containing oxygen at 100%, a release of Znfrom the film can be suppressed at a deposition temperature even higherthan or equal to 300° C.

It is preferable that the oxide semiconductor film be the one which ishighly purified and hardly contain impurities such as copper, aluminum,and chlorine. In the process for manufacturing the transistor, steps inwhich these impurities are not mixed in the oxide semiconductor film orattached to the surface of the oxide semiconductor film are preferablyselected as appropriate. In the case where the impurities are attachedto the surface of the oxide semiconductor film, the impurities on thesurface of the oxide semiconductor film are preferably removed byexposure to oxalic acid or dilute hydrofluoric acid or plasma treatment(such as N₂O plasma treatment). Specifically, the concentration ofcopper in the oxide semiconductor film is lower than or equal to 1×10¹⁸atoms/cm³, preferably lower than or equal to 1×10¹⁷ atoms/cm³. Further,the concentration of aluminum in the oxide semiconductor film is smallerthan or equal to 1×10¹⁸ atoms/cm³. Further, the concentration ofchlorine in the oxide semiconductor film is smaller than or equal to2×10¹⁸ atoms/cm³.

The oxide semiconductor film is preferably highly purified by sufficientremoval of impurities such as hydrogen or sufficient supply of oxygen tobe in a supersaturated state. Specifically, the concentration ofhydrogen in the oxide semiconductor film is 5×10¹⁹ atoms/cm³ or less,preferably 5×10¹⁸ atoms/cm³ or less, further preferably 5×10¹⁷ atoms/cm³or less. Note that the concentration of hydrogen in the oxidesemiconductor film is measured by secondary ion mass spectrometry(SIMS). Further, for sufficient supply of oxygen to make the film in asupersaturated state, an insulating film (e.g., SiO_(X)) containingexcess oxygen is provided to be in contact with and covers the oxidesemiconductor film.

Next, a gate insulating film 412 is formed over the base insulating film432 and the oxide semiconductor film 403 (see FIG. 2B). Note that thegate insulating film 412 may be provided at least between the gateelectrode 401 a to be formed later and the oxide semiconductor film 403.

As a material for the gate insulating film 412, silicon oxide, galliumoxide, aluminum oxide, zirconium oxide, yttrium oxide, hafnium oxide,lanthanum oxide, neodymium oxide, tantalum oxide, silicon nitride,silicon oxynitride, aluminum oxynitride, silicon nitride oxide, or thelike can be used.

As the gate insulating film 412, an insulating film which releasesoxygen by heat treatment at a temperature higher than or equal to 250°C. and lower than or equal to 700° C., preferably higher than or equalto 300° C. and lower than or equal to 450° C. may be used.

In a transistor including an oxide semiconductor film, oxygen vacanciesin the oxide semiconductor film serve as donors, which cause a shift ofthe threshold voltage of the transistor in the negative direction.Oxygen vacancies at an interface between a gate insulating film and theoxide semiconductor film are a major factor of change in electriccharacteristics of the transistor, because electrons are captured due toan operation of the transistor or the like. Therefore, reduction inoxygen vacancies in the oxide semiconductor film and at the interfacebetween the oxide semiconductor film and the gate insulating film leadsto stable electric characteristics of the transistor including the oxidesemiconductor film and improvement in reliability. Therefore, whenoxygen is released from the gate insulating film, oxygen vacancies inthe oxide semiconductor film and at the interface between the oxidesemiconductor film and the gate insulating film can be reduced.

Next, the substrate 400 over which the gate insulating film 412 isformed may be subjected to heat treatment for removing moisture,hydrogen, and the like.

For the heat treatment, an electric furnace or a device for heating anobject by heat conduction or heat radiation from a heating element suchas a resistance heating element can be used. For example, a rapidthermal annealing (RTA) apparatus such as a lamp rapid thermal annealing(LRTA) apparatus or a gas rapid thermal annealing (GRTA) apparatus canbe used. An LRTA apparatus is an apparatus for heating an object to beprocessed by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA treatment may be performed asfollows. The object to be processed is put in a heated inert gasatmosphere, heated for several minutes, and taken out of the inert gasatmosphere. The GRTA treatment enables high-temperature heat treatmentfor a short time. Moreover, the GRTA treatment can be employed even whenthe temperature exceeds the upper temperature limit of the object to beprocessed. Note that the inert gas may be changed during the treatmentto a gas including oxygen. The heat treatment is performed in anatmosphere including oxygen, whereby the defect density in the film canbe decreased.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not include moisture, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into the heat treatment apparatusis higher than or equal to 6N (99.9999%), preferably higher than orequal to 7N (99.99999%) (that is, the impurity concentration is lowerthan or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

In the case where the mother glass is used as the substrate 400, highprocess temperature and a long period of process time drastically shrinkthe mother glass; therefore, the temperature of the heat treatment ishigher than or equal to 200° C. and lower than or equal to 450° C.,preferably higher than or equal to 250° C. and lower than or equal to350° C.

Impurities such as moisture or hydrogen in the gate insulating film 412can be removed by the heat treatment. Further, by the heat treatment,the defect density in the films can be reduced. The impurities or defectdensity in the gate insulating film 412 is reduced, whereby the electriccharacteristics of the transistor can be improved and the change in theelectric characteristics of the transistor due to the operation of thetransistor or the like can be suppressed.

The above heat treatment can be referred to as dehydration treatment,dehydrogenation treatment, or the like because of its advantageouseffect of removing moisture, hydrogen, or the like. Such dehydrationtreatment or dehydrogenation treatment may be performed once or pluraltimes.

Then, a conductive film 401 is formed over the gate insulating film 412(see FIG. 2B).

The conductive film 401 may be formed to have a single-layer orstacked-layer structure using one or more of Al, Ti, Cr, Co, Ni, Cu, Y,Zr, Mo, Ag, Ta, and W, a nitride of any of these elements, an oxide ofany of these elements, and an alloy of any of these elements.Alternatively, an oxide or an oxynitride which contains at least In andZn may be used. For example, an In—Ga—Zn—O—N-based material can be used.In this embodiment, a tantalum nitride film having a thickness of 30 nmis formed and a tungsten film having a thickness of 200 nm is formedover the tantalum nitride film.

A resist mask which is to be formed later by the exposure to an electronbeam is thin and it is difficult to form a pattern of the conductivefilm 401 in some cases. Therefore, a hard mask film 408 is formed overthe conductive film 401, a hard mask film 409 is formed over the hardmask film 408, and the hard masks are used as a mask (see FIG. 2B).

It is preferable that the hard mask film 408 not be easily etched underconditions where the conductive film 401 is etched, because the hardmask film 408 is used as a mask when the conductive film 401 is etched.As the hard mask film 408, a silicon oxide film or a silicon nitrideoxide film is preferably used.

Further, it is preferable that the hard mask film 409 be not easilyetched under conditions where the hard mask film 408 is etched, becausethe hard mask film 409 is used as a mask when the hard mask film 408 isetched. As the hard mask film 409, an amorphous silicon film ispreferably used.

Etching selectivity of the hard mask film 409 to a resist mask formed bythe exposure to the electron beam is high; therefore it is easy to forma pattern even if the resist mask is thin. Further, etching selectivityof the hard mask film 408 to the hard mask film 409 and etchingselectivity of the conductive film 401 (the tungsten film of the upperlayer in this embodiment) to the hard mask film 408 are high; therefore,it is easy to form pattern of the lower layer by using the patternedfilm thereover as a mask.

Next, a resist is formed over the hard mask film 409 and subjected toexposure to an electron beam; thus, a resist mask 420 is formed (seeFIG. 2B).

In an electron beam writing apparatus capable of electron beamirradiation, the acceleration voltage is preferably in the range from 5kV to 50 kV, for example. The current intensity is preferably in therange from 5×10⁻¹² A to 1×10⁻¹¹ A. The minimum beam size is preferably 2nm or less. The minimum possible pattern line width is preferably 8 nmor less.

Under the above conditions, the resist mask 420 with a width of, forexample, 1 nm or more and 30 nm or less, preferably 20 nm or less, morepreferably 8 nm or less, can be obtained.

For the exposure to an electron beam, the resist mask 420 is preferablyas thin as possible. When the resist mask 420 is thin, a surface onwhich the resist mask is formed is preferably as flat as possible. Inthe method for manufacturing the semiconductor device of thisembodiment, the unevenness due to the base insulating film 432 and thelike can be reduced by planarization treatment such as a polishingtreatment (i.e. CMP treatment), etching (dry etching or wet etching)treatment, plasma treatment, or the like of the base insulating film 432and the like; thus, the resist mask can be thin. This facilitates theexposure to an electron beam.

Next, the hard mask film 409 is selectively etched and a hard mask film409 a is formed (see FIG. 2C). Further, the resist mask 420 is removedafter etching. In this embodiment, the resist mask 420 is removed;however the present invention is not limited thereto. The resist mask420 is almost eliminated; therefore, it may be left.

Next, the hard mask film 408 is selectively etched with the use of thehard mask film 409 a as a mask, and a hard mask film 408 a is formed(see FIG. 2D). Further, the hard mask film 409 a is removed afteretching. The hard mask film 409 a is not necessarily removed similar tothe resist mask 420; it may be left.

Then, the conductive film 401 is etched with the use of the hard maskfilm 408 a as a mask, and the gate electrode 401 a is formed (see FIG.3A). Further, the hard mask film 408 a may be removed after etching.Here, a region in the oxide semiconductor film 403 over which the gateelectrode 401 a is formed will become a channel formation region of thetransistor 450. Since the channel length L can be determined by theexposure to an electron beam, a transistor with a small channel length,e.g., a channel length greater than or equal to 1 nm and less than orequal to 30 nm, can be manufactured.

Next, treatment for adding an impurity 421 to the oxide semiconductorfilm 403 is performed with the use of the gate electrode 401 a as amask, so that the low-resistance region 403 a, the low-resistance region403 b, and the channel formation region 403 c may be formed in aself-aligned manner (see FIG. 3A).

As the impurity 421 to be added is phosphorus, boron, nitrogen, arsenic,argon, aluminum, indium, a molecular ion containing any of the aboveelement, or the like is used. As a method for adding the impurity 421 tothe oxide semiconductor film 403, an ion doping method or an ionimplantation method can be used.

The treatment for adding the impurity 421 to the oxide semiconductorfilm 403 may be performed plural times. In the case where the treatmentfor adding the impurity 421 to the oxide semiconductor film 403 isperformed plural times, the kind of the impurity 421 may be the same inthe plural treatments or different in every treatment.

The dose of the impurity 421 is preferably 1×10¹³ ions/cm² to 5×10¹⁶ions/cm². When phosphorus is added as the impurity, the accelerationvoltage is preferably 0.5 kV to 80 kV. In this embodiment, phosphorus isadded to the oxide semiconductor film 403 as the impurity 421 by an ionimplantation method under the conditions where the acceleration voltageis 30 kV and the dose is 1.0×10¹⁵ ions/cm².

The low-resistance region 403 a and the low-resistance region 403 b eachhave a higher impurity concentration than the channel formation region403 c. When the impurity concentration is increased, the carrier densityof the oxide semiconductor film is increased and contact resistancebetween the source and drain electrodes and the oxide semiconductor filmis reduced; thus, favorable ohmic contact can be obtained between thesource and drain electrodes and the oxide semiconductor film.

Next, the insulating film 415 is formed over the gate insulating film412 and the gate electrode 401 a (see FIG. 3B).

As a material for the insulating film 415, silicon oxide, gallium oxide,aluminum oxide, zirconium oxide, yttrium oxide, hafnium oxide, lanthanumoxide, neodymium oxide, tantalum oxide, silicon nitride, siliconoxynitride, aluminum oxynitride, silicon nitride oxide, or the like canbe used. Note that the insulating film 415 may have a single-layerstructure or a stacked-layer structure.

Moreover, an aluminum oxide film is preferably provided in theinsulating film 415 which is in contact with the gate insulating film412 and the gate electrode 401 a. In particular, it is preferable to usean aluminum oxide film with a film density of higher than or equal to3.2 g/cm³, preferably higher than or equal to 3.6 g/cm³. The aluminumoxide film has a thickness greater than or equal to 30 nm and less thanor equal to 150 nm, preferably greater than or equal to 50 nm and lessthan or equal to 100 nm. When the film density of the aluminum oxidefilm is higher than or equal to the above mentioned density, moisture orhydrogen can be prevented from entering and diffusing into the oxidesemiconductor film. In addition, release of oxygen from the oxidesemiconductor film 403 can be suppressed.

Next, removing (polishing) treatment is performed on a part of theinsulating film 415 while the gate electrode 401 a is not exposed and aninsulating film 415 a is formed (see FIG. 3C). In this embodiment, theremoving treatment is performed so that the insulating film 415 a havinga thickness of 100 nm is formed over the gate electrode 401 a.

For the removing treatment, chemical mechanical polishing (CMP)treatment can be preferably used.

Note that the CMP treatment is used as the removing treatment in thisembodiment; however, another removing treatment may be used.Alternatively, the polishing treatment such as the CMP treatment may becombined with etching (dry etching or wet etching) treatment or plasmatreatment. When the removing treatment is combined with etchingtreatment, plasma treatment, or the like, the order of steps is notparticularly limited and may be set as appropriate depending on thematerials, the film thicknesses, and the surface roughness of theinsulating film 415. Alternatively, a large part of the insulating film415 may be removed by CMP treatment and other part of the insulatingfilm 415 may be removed by dry etching treatment.

Note that the CMP treatment may be performed only once or plural times.When the CMP treatment is performed plural times, first polishing ispreferably performed with a high polishing rate followed by finalpolishing with a low polishing rate. By performing polishing steps withdifferent polishing rates in combination, the planarity of the surfaceof the insulating film 415 can be further improved.

With the above-described structure, the height of the gate electroderegion can become high by using the insulating film 415 a, a sidewallinsulating film can be formed on a side surface of a gate electrode 401a later, and the source electrode and the drain electrode can be easilyseparated from each other.

Next, an anti-reflective film 407 is formed over the insulating film 415a, a resist is formed over the anti-reflective film 407, and a resistmask 430 overlapping with the low-resistance regions 403 a and 403 b,and the channel formation region 403 c is selectively formed by exposingthe resist to an electron beam (see FIG. 3D).

The anti-reflective film 407 may be formed to have a single-layer orstacked-layer structure using one or more of Al, Ti, Cr, Co, Zr, Mo, Ta,and W, a nitride of any of these elements, an oxide of any of theseelements, and an alloy of any of these elements. Alternatively, an oxideor an oxynitride which contains at least In and Zn may be used. Forexample, an In—Ga—Zn—O—N-based material can be used. In this embodiment,as the anti-reflective film 407, a tungsten film having a thickness of30 nm is formed.

The anti-reflective film 407 is provided under the resist so as not toreflect or transmit the light in exposing. By providing ananti-reflective film under the resist, accuracy of forming a patternafter the exposure and development can be improved.

Further, a hard mask film (a stacked film in which an amorphous siliconfilm is stacked over a silicon nitride oxide film or a silicon oxidefilm) is preferably formed over the anti-reflective film 407 similar tothe formation of the gate electrode 401 a. With such a structure,etching selectivity of the anti-reflective film 407 to the resist maskis high even if the resist mask is thin; therefore, it is easy to form apattern of a lower layer using a patterned film thereover as a mask.

Further, the conditions for the resist mask 420 can be referred to forthe conditions for the exposure to an electric beam.

Next, the anti-reflective film 407, the insulating film 415 a, and thegate insulating film 412 are etched, whereby an anti-reflective film 407a, the insulating film 415 b, and the gate insulating film 412 a eachhaving an island shape are formed (see FIG. 4A). Here, the distance L1in the drawing can be determined by the exposure to an electric beam;therefore, a resistance between the gate electrode 401 a and a region(contact region) in which the oxide semiconductor film 403 is in contactwith the source electrode 405 a (or the drain electrode 405 b), which isformed later, is reduced and on-state characteristics of the transistor450 can be improved. For example, a transistor in which the distance L1in the drawing is less than or equal to 30 nm can be manufactured.

Next, the resist mask 430 is removed and a conductive film 405 is formedover the base insulating film 432, the low-resistance region 403 a, thelow-resistance region 403 b, and the anti-reflective film 407 a (seeFIG. 4B).

The conductive film 405 may be formed to have a single-layer orstacked-layer structure using one or more of Al, Ti, Cr, Co, Ni, Cu, Y,Zr, Mo, Ag, Ta, Ru, and W, a nitride of any of these elements, an oxideof any of these elements, and an alloy of any of these elements.Alternatively, an oxide or an oxynitride which contains at least In andZn may be used. For example, an In—Ga—Zn—O—N-based material can be used.In this embodiment, a tungsten film with a thickness of 30 nm is formed.

Next, an insulating film 425 is formed over the conductive film 405 (seeFIG. 4C).

As a material for the insulating film 425, silicon oxide, gallium oxide,aluminum oxide, zirconium oxide, yttrium oxide, hafnium oxide, lanthanumoxide, neodymium oxide, tantalum oxide, silicon nitride, siliconoxynitride, aluminum oxynitride, silicon nitride oxide, or the like canbe used. The insulating film 425 may have a single-layer structure or astacked-layer structure.

Moreover, an aluminum oxide film is preferably provided in theinsulating film 425 which is in contact with the conductive film 405. Inparticular, it is preferable to use an aluminum oxide film with a filmdensity of higher than or equal to 3.2 g/cm³, preferably higher than orequal to 3.6 g/cm³. The aluminum oxide film has a thickness greater thanor equal to 30 nm and less than or equal to 150 nm, preferably greaterthan or equal to 50 nm and less than or equal to 100 nm. When the filmdensity of the aluminum oxide film is within the above range, moistureor hydrogen can be prevented from entering and diffusing into the oxidesemiconductor film. In addition, release of oxygen from the oxidesemiconductor film 403 can be suppressed.

Next, removing (polishing) treatment is performed on parts of theinsulating film 425 and the conductive film 405, and the anti-reflectivefilm 407 a so that the insulating film 415 b is exposed, and theinsulating film 425 a, the source electrode 405 a, and the drainelectrode 405 b are formed by processing the insulating film 425 and theconductive film 405 (see FIG. 4D).

For the removing treatment, chemical mechanical polishing (CMP)treatment can be preferably used.

Note that in this embodiment, the heights of top surfaces of the sourceelectrode 405 a and the drain electrode 405 b are substantially the sameas the heights of top surfaces of the insulating film 415 b and theinsulating film 425 a. Note that, “the heights of top surfaces of thesource electrode 405 a and the drain electrode 405 b are substantiallythe same as the heights of top surfaces of the insulating film 415 b andthe insulating film 425 a” in this embodiment includes the case wherethe difference between the heights of top surfaces of the sourceelectrode 405 a and the drain electrode 405 b and the heights of topsurfaces of the insulating film 415 b and the insulating film 425 a iswithin 10% of the height of the gate electrode region and less than orequal to 20 nm. With such a structure, coverage of a thin film formed ina later step (a manufacturing step or the like of a semiconductor deviceor an electronic device including the transistor 450) can be improved,so that disconnection of a thin film or a wiring can be prevented. Forexample, if there is a step between the top surfaces of the sourceelectrode 405 a and the drain electrode 405 b and the top surfaces ofthe insulating film 415 b and the insulating film 425 a, a film or awiring over the step is cut and the defect occurs; however, if the topsurfaces of the source electrode 405 a and the drain electrode 405 b andthe top surfaces of the insulating film 415 b and the insulating film425 a are substantially the same, such a defect can be prevented and thereliability can be improved.

Further, if the step is slight as coverage of a film or a wiring formedin a later process is not impaired, there is no problem even if theheights of top surfaces of the source electrode 405 a and the drainelectrode 405 b are slightly different from the heights of top surfacesof the insulating film 415 b and the insulating film 425 a.

Note that the CMP treatment is used as the removing treatment in thisembodiment; however, another removing treatment may be used.Alternatively, the polishing treatment such as the CMP treatment may becombined with etching (dry etching or wet etching) treatment or plasmatreatment. In the case where the removing treatment is combined withetching treatment, plasma treatment or the like, the order of the stepsis not particularly limited, and may be set as appropriate depending onthe material, thickness, and roughness of the surface of the insulatingfilm 425. Alternatively, a large part of the insulating film 425 may beremoved by CMP treatment and other part of the insulating film 425 maybe removed by dry etching treatment.

Note that the CMP treatment may be performed only once or plural times.When the CMP treatment is performed plural times, first polishing ispreferably performed with a high polishing rate followed by finalpolishing with a low polishing rate. By performing polishing steps withdifferent polishing rates in combination, the planarity of the surfaceof the insulating film 425 can be further improved.

As described above, the removing treatment is performed so that theinsulating film 415 b is exposed, whereby the source electrode 405 a andthe drain electrode 405 b can be formed.

With the above-described structure, the height of the gate electroderegion can become high by using the insulating film 415 b and a sidewallinsulating film can be formed on a side surface of a gate electrode 401a at the same time. Thus, when removing (polishing) treatment isperformed on a conductive film which is to be a source electrode and adrain electrode, the conductive film is processed, and the sourceelectrode 405 a and the drain electrode 405 b can be easily separatedfrom each other.

Further, the length of the gate electrode 401 a in the channel lengthdirection can be determined by the exposure to an electron beam. Here, aregion of the oxide semiconductor film 403, which overlaps with the gateelectrode 401 a becomes a channel formation region of the transistor.That is, since the channel length L can be determined by the exposure toan electron beam, a transistor with a small channel length can bemanufactured.

Further, the source electrode 405 a and the drain electrode 405 b areprovided in contact with the exposed portion of a top surface of theoxide semiconductor film 403 and the insulating film 415 b. Therefore,the distance L1 between the gate electrode 401 a and the region (contactregion) in which the oxide semiconductor film 403 is in contact with thesource electrode 405 a (or the drain electrode 405 b) can be determinedby the exposure to an electron beam, so that the resistance between thegate electrode 401 a and the region (contact region) in which the oxidesemiconductor film 403 is in contact with the source electrode 405 a (orthe drain electrode 405 b) is reduced; thus, the on-statecharacteristics of the transistor 450 can be improved.

Therefore, a semiconductor device which is miniaturized and has goodelectrical characteristics and a method for manufacturing thesemiconductor device can be provided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, a structure of a semiconductor device in oneembodiment of the present invention, which is different from that inEmbodiment 1, will be described.

FIGS. 5A and 5B are a plan view and a cross-sectional view of atransistor 470.

FIG. 5A is a plan view and FIG. 5B is a cross-sectional view taken alongline C-D in FIG. 5A. Note that in FIG. 5A, some components (e.g., a baseinsulating film 432) of the transistor 470 are omitted to avoidcomplexity.

Note that, in this embodiment, portions that are similar to the portionsin Embodiment 1 are denoted by the same reference numerals in thedrawings, and detailed description thereof is omitted.

<Structure of Semiconductor Device According to this Embodiment>

FIGS. 5A and 5B are an example of a structure of a semiconductor devicemanufactured according to a method of this embodiment. The transistor470 illustrated in FIGS. 5A and 5B includes a base insulating film 432provided over a substrate 400 having an insulating surface, an oxidesemiconductor film 403 including a low-resistance region 403 a, achannel formation region 403 c surrounding the low-resistance region 403a, and a low-resistance region 403 b surrounding the channel formationregion 403 c over the base insulating film 432, a gate insulating film412 a over the oxide semiconductor film 403, a gate electrode 401 aprovided over the gate insulating film 412 a and overlapping with thechannel formation region 403 c, an insulating film 415 b over the gateinsulating film 412 a and the gate electrode 401 a, a source electrode405 a overlapping with a part of the low-resistance region 403 a, adrain electrode 405 b overlapping with parts of the base insulating film432 and the low-resistance region 403 b, an insulating film 425 a overthe source electrode 405 a and the drain electrode 405 b, an interlayerinsulating film 427 over the insulating film 415 b, the insulating film425 a, the source electrode 405 a, and the drain electrode 405 b, and awiring layer 431 a, a wiring layer 431 b, and a wiring layer 431 c whichare electrically connected to the gate electrode 401 a, the sourceelectrode 405 a, and the drain electrode 405 b, respectively, throughopenings provided in the insulating film 415 b and the insulating film425 a.

In the drawings, although the semiconductor device has a structure inwhich the source electrode 405 a is provided in the center of thesemiconductor device and the gate electrode 401 a and the drainelectrode 405 b are provided in the periphery of the source electrode405 a, the structure of a semiconductor device is not limited to this.An arrangement of the components can be changed as appropriate as longas a function of the semiconductor device is not negatively affected.

<Manufacturing Method of Semiconductor Device According to thisEmbodiment>

A method for manufacturing the transistor 470 will be described. Notethat the description of points similar to that in Embodiment 1 isomitted.

The substrate 400, the base insulating film 432, the oxide semiconductorfilm 403, and the gate insulating film (which is to be the gateinsulating film 412 a) which are included in the transistor 470 each canbe formed using a material and a method similar to those in Embodiment1.

After the gate insulating film is formed, a conductive film which is tobe the gate electrode 401 a is formed over the gate insulating film. Theconductive film can be formed using a material and a method similar tothose in Embodiment 1.

A hard mask film and a resist are formed in this order over theconductive film to be the gate electrode 401 a and the resist ispatterned through exposure to an electron beam; thus, a mask is formed.Further, the hard mask film may be a single layer or a stacked layer.The hard mask film can be formed using a material and a method similarto those in Embodiment 1.

The hard mask film is selectively etched using the mask formed of theresist; thus, an island-shaped hard mask film is formed. Moreover, theconductive film is selectively etched using the island-shaped hard maskfilm as a mask; thus, the gate electrode 401 a is formed. Here, a regionof the oxide semiconductor film 403 overlapping with the gate electrode401 a will become a channel formation region of the transistor 470.Since the channel length L can be determined by the exposure to anelectron beam, a transistor with a small channel length, e.g., a channellength greater than or equal to 1 nm and less than or equal to 30 nm,can be manufactured.

The channel length of the transistor 470 is preferably equal in any partof the transistor. Since the shape of the channel formation region ofthe transistor of this embodiment includes a curved line, it ispreferable to form the curved line by exposure to an electron beam so asto be smooth and so as to have an equal line width.

In order to form a smooth curved line with an equal line width byexposure to an electron beam, there is a method for exposure of a curvedline by rotating a stage overlapping with a substrate thereon, forexample. With a linearly movable stage, a resist mask can also bepatterned so that the channel length of the transistor becomes equal, byusing a method in which the size or direction of a figure for dividingelectron beam writing regions is optimized in accordance with thepattern of the electron beam, a multi-pass writing method in which afigure is shifted by a uniform width and writing is performed with anoverlap so that the amount of light exposure of a pattern becomes equal,or the like. It is preferable to use the above method or the like toform a resist mask with an equal line width so that the channel lengthof the transistor 470 becomes equal.

After the gate electrode 401 a is formed, an insulating film (which isto be the insulating film 415 b) is formed over the gate insulating filmand the gate electrode 401 a. The insulating film can be formed using amaterial and a method similar to that in Embodiment 1.

Next, removing (polishing) treatment is performed on a part of theinsulating film, an anti-reflective film which prevents reflection ofthe light in exposing and a resist are formed over the insulating filmon which the removing treatment has been performed, and a sidewallinsulating film is formed on a side surface of a gate electrode 401 a byetching. At the same time, the height of the gate electrode regionbecomes high by using the insulating film which forms the sidewallinsulating film. The removing treatment can be performed in a mannersimilar to that in Embodiment 1, and the anti-reflective film and theresist can be formed each using a material and a method similar to thosein Embodiment 1.

Next, a conductive film which is to be the source electrode 405 a andthe drain electrode 405 b is formed over the base insulating film 432,the low-resistance region 403 a, and the low-resistance region 403 b,which have been exposed by etching, and the anti-reflective film and aninsulating film (which is to be the insulating film 425 a) is formedover the conductive film. The anti-reflective film and the insulatingfilm can be formed each using a material and a method similar to thosein Embodiment 1.

Then, removing (polishing) treatment is performed until theanti-reflective film is removed completely, so that the source electrode405 a and the drain electrode 405 b are formed. The insulating film 425a is formed at the same time.

Here, the source electrode 405 a and the drain electrode 405 b areprovided in contact with a top surface of the oxide semiconductor film403 which is exposed and the insulating film 415 b. Therefore, thedistance L1 between the gate electrode 401 a and the region (contactregion) in which the oxide semiconductor film 403 is in contact with thesource electrode 405 a (or the drain electrode 405 b) can be determinedby the exposure to an electron beam, so that the resistance between thegate electrode 401 a and the region (contact region) in which the oxidesemiconductor film 403 is in contact with the source electrode 405 a (orthe drain electrode 405 b) is reduced; thus, the on-statecharacteristics of the transistor 470 can be improved.

Next, the interlayer insulating film 427 is formed over the insulatingfilm 415 b, the insulating film 425 a, the source electrode 405 a, andthe drain electrode 405 b, the insulating film 415 b, the insulatingfilm 425 a, and the interlayer insulating film 427 are etched; thus,openings reaching the gate electrode 401 a, the source electrode 405 a,and the drain electrode 405 b, respectively, are formed.

Then, a conductive film is formed in the openings and over theinterlayer insulating film 427 and the conductive film is etched,whereby the wiring layer 431 a, the wiring layer 431 b, and the wiringlayer 431 c which are electrically connected to the gate electrode 401a, the source electrode 405 a, and the drain electrode 405 b,respectively, can be formed.

In the transistor 470 described in this embodiment, the length of thegate electrode 401 a in the channel length direction is determined byusing the resist obtained by the exposure to an electric beam as a mask.Precise exposure and development using an electron beam can provide aprecise pattern.

With the above-described structure, the height of the gate electroderegion can become high by using the insulating film 415 b and a sidewallinsulating film can be formed on a side surface of a gate electrode 401a at the same time. Thus, when removing (polishing) treatment isperformed on a conductive film which is to be a source electrode and adrain electrode, the source electrode 405 a and the drain electrode 405b can be easily separated from each other.

Further, the length of the gate electrode 401 a in the channel lengthdirection can be determined by the exposure to an electron beam. Here, aregion of the oxide semiconductor film 403, which overlaps with the gateelectrode 401 a becomes a channel formation region of the transistor.That is, since the channel length L can be determined by the exposure toan electron beam, a transistor with a small channel length can bemanufactured.

Further, the source electrode 405 a and the drain electrode 405 b areprovided in contact with the exposed portion of a top surface of theoxide semiconductor film 403 and the insulating film 415 b. Therefore,the distance L1 between the gate electrode 401 a and the region (contactregion) in which the oxide semiconductor film 403 is in contact with thesource electrode 405 a (or the drain electrode 405 b) can be determinedby the exposure to an electron beam, so that the resistance between thegate electrode 401 a and the region (contact region) in which the oxidesemiconductor film 403 is in contact with the source electrode 405 a (orthe drain electrode 405 b) is reduced; thus, the on-statecharacteristics of the transistor 470 can be improved.

Furthermore, in the transistor 470, only one of the source and drainelectrode is connected to the end portion of the oxide semiconductorfilm which tends to become less resistive; thus, a transistor in which aparasitic channel is not easily formed and which has excellentelectrical characteristics can be provided.

Therefore, a semiconductor device which is miniaturized and has goodelectrical characteristics and a method for manufacturing thesemiconductor device can be provided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

In this embodiment, an example of a semiconductor device which includesthe transistor described in Embodiment 1, can hold stored data even whennot powered, and does not have a limitation on the number of writecycles will be described with reference to drawings. Note that atransistor 162 included in the semiconductor device in this embodimentis the transistor 450 described in Embodiment 1.

FIGS. 6A to 6C illustrate an example of a structure of a semiconductordevice. FIG. 6A is a cross-sectional view of the semiconductor device,FIG. 6B is a plan view of the semiconductor device, and FIG. 6C is acircuit diagram of the semiconductor device. Here, FIG. 6A correspondsto cross sections taken along line E-F and line G-H in FIG. 6B.

The semiconductor device illustrated in FIGS. 6A and 6B includes atransistor 160 including a first semiconductor material in a lowerportion, and the transistor 162 including a second semiconductormaterial in an upper portion. The transistor 162 has the same structureas the transistor 450 described in Embodiment 1.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material may be a semiconductormaterial other than an oxide semiconductor (e.g., silicon) and thesecond semiconductor material may be an oxide semiconductor. Atransistor including a material other than an oxide semiconductor canoperate at high speed easily. On the other hand, a transistor includingan oxide semiconductor enables charge to be held for a long time owingto its characteristics.

The transistor 162 includes an oxide semiconductor and thus has smalloff-state current; thus, the use of the transistor 162 enables storeddata to be held for a long time. In other words, a semiconductor devicein which refresh operation is not needed or the frequency of refreshoperation is extremely low can be provided, which results in asufficient reduction in power consumption.

Although all the transistors are n-channel transistors here, p-channeltransistors can also be used. The technical feature of the disclosedinvention is to use an oxide semiconductor in the transistor 162 so thatdata can be held; therefore, it is not necessary to limit a specificstructure of the semiconductor device, such as a material of thesemiconductor device or a structure of the semiconductor device, to thestructure described here.

The transistor 160 in FIG. 6A includes a channel formation region 116provided in a substrate 100 including a semiconductor material (e.g.,silicon), impurity regions 120 provided such that the channel formationregion 116 is sandwiched therebetween, intermetallic compound regions124 in contact with the impurity regions 120, a gate insulating film 108provided over the channel formation region 116, and a gate electrode 110provided over the gate insulating film 108. Note that a transistor whosesource electrode and drain electrode are not illustrated in a drawingmay be referred to as a transistor for convenience. Further, in such acase, in description of a connection of a transistor, a source regionand a source electrode may be collectively referred to as a “sourceelectrode,” and a drain region and a drain electrode may be collectivelyreferred to as a “drain electrode.” That is, in this specification, theterm “source electrode” may include a source region.

An element isolation insulating film 106 is formed over the substrate100 so that the transistor 160 is surrounded by the element isolationinsulating film 106. An insulating film 130 is formed so that thetransistor 160 is covered with the insulating film 130. Note that forhigher integration, the transistor 160 preferably has a structurewithout a sidewall insulating film as illustrated in FIG. 6A. On theother hand, when the characteristics of the transistor 160 havepriority, the sidewall insulating films may be formed on side surfacesof the gate electrode 110, so that the impurity regions 120 each includeregions having different impurity concentrations.

The transistor 162 illustrated in FIG. 6A includes an oxidesemiconductor in the channel formation region. An oxide semiconductorfilm 144 includes low-resistance regions 144 a and 144 b and a channelformation region 144 c and a gate insulating layer 146 is formed overthe oxide semiconductor film. The channel formation region 144 c issandwiched between the low-resistance regions 144 a and 144 b.

In a manufacturing step of the transistor 162, an insulating film 135including a sidewall insulating film on a side surface of a gateelectrode 148 is formed in a step of removing the insulating filmprovided over the gate electrode 148 by chemical mechanical polishingtreatment. Further, at the same time, the height of the gate electroderegion can become high by using the insulating film which forms thesidewall insulating film.

Therefore, in the transistor 162, the height of the gate electroderegion can become high, whereby when the removing (polishing) treatmentis performed on a conductive film which is to be a source electrode anda drain electrode, the source electrode and the drain electrode can beeasily separated from each other.

Further, the length of the gate electrode 148 in the channel lengthdirection can be determined by the exposure to an electron beam. Here, aregion of the oxide semiconductor film, which overlaps with the gateelectrode 148 becomes a channel formation region of the transistor. Thatis, since the channel length L can be determined by the exposure to anelectron beam, a transistor with a small channel length, e.g., a channellength greater than or equal to 1 nm and less than or equal to 30 nm,can be manufactured.

Further, the source electrode 142 a and the drain electrode 142 b areprovided in contact with the exposed portion of a top surface of theoxide semiconductor film 144 and the insulating film 135. Therefore, thedistance between the gate electrode 148 and the region (contact region)in which the oxide semiconductor film 144 is in contact with the sourceelectrode 142 a (or the drain electrode 142 b) can be determined by theexposure to an electron beam, so that the resistance between the gateelectrode 148 and the region (contact region) in which the oxidesemiconductor film 144 is in contact with the source electrode 142 a (orthe drain electrode 142 b) is reduced; thus, the on-statecharacteristics of the transistor 162 can be improved. For example, atransistor in which a distance between the gate electrode 148 and aregion (contact region) in which the oxide semiconductor film 144 is incontact with the source electrode 142 a (or the drain electrode 142 b)is greater than or equal to 1 nm and less than or equal to 30 nm can bemanufactured.

An interlayer insulating film 149 and an insulating film 150 each havinga single-layer structure or a stacked-layer structure are provided overthe transistor 162. In this embodiment, an aluminum oxide film is usedas the insulating film 150. When the aluminum oxide film has highdensity (film density of 3.2 g/cm³ or more, preferably 3.6 g/cm³ ormore), the transistor 162 can have stable electric characteristics.

Further, a conductive film 153 is provided in a region overlapping withthe source electrode 142 a with the interlayer insulating film 149 andthe insulating film 150 provided therebetween. The source electrode 142a, the interlayer insulating film 149, the insulating film 150, and theconductive film 153 constitute a capacitor 164. That is, the sourceelectrode 142 a functions as one electrode of the capacitor 164 and theconductive film 153 functions as the other electrode of the capacitor164. Note that the capacitor 164 may be omitted if a capacitor is notneeded. Alternatively, the capacitor 164 may be separately providedabove the transistor 162.

An insulating film 152 is provided over the transistor 162 and thecapacitor 164. Further, wirings 156 a and 156 b for connecting thetransistor 162 to another transistor are provided over the insulatingfilm 152. The wiring 156 a is electrically connected to the sourceelectrode 142 a through the electrode formed in an opening formed in theinterlayer insulating film 149, the insulating film 150, and theinsulating film 152. The wiring 156 b is electrically connected to thesource electrode 142 b through the electrode formed in an opening formedin the interlayer insulating film 149, the insulating film 150, and theinsulating film 152.

In FIGS. 6A and 6B, the transistor 160 is provided so as to overlap withat least part of the transistor 162. The source region or the drainregion of the transistor 160 is preferably provided so as to overlapwith part of the oxide semiconductor film 144. Further, the transistor162 and the capacitor 164 are provided so as to overlap with at leastpart of the transistor 160. For example, the conductive film 153 of thecapacitor 164 is provided so as to overlap with at least part of thegate electrode 110 of the transistor 160. When such a planar layout isemployed, the area occupied by the semiconductor device can be reduced;thus, the degree of integration can be increased.

FIG. 6C illustrates an example of a circuit configuration correspondingto FIGS. 6A and 6B.

In FIG. 6C, a first wiring (1st Line) is electrically connected to asource electrode of the transistor 160. A second wiring (2nd Line) iselectrically connected to a drain electrode of the transistor 160. Athird wiring (3rd Line) is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 162. A fourth wiring(4th Line) is electrically connected to a gate electrode of thetransistor 162. A gate electrode of the transistor 160 and the other ofthe source electrode and the drain electrode of the transistor 162 areelectrically connected to one electrode of the capacitor 164. A fifthwiring (5th Line) is electrically connected to the other electrode ofthe capacitor 164.

The semiconductor device in FIG. 6C utilizes a characteristic in whichthe potential of the gate electrode of the transistor 160 can be held,and thus can write, hold, and read data as described below.

Writing and holding of data will be described. First, the potential ofthe fourth wiring is set to a potential at which the transistor 162 isturned on, so that the transistor 162 is turned on. Thus, the potentialof the third wiring is supplied to a node (node FG) to which the gateelectrode of the transistor 160 and the capacitor 164 are connected. Inother words, predetermined charge is supplied to the node FG (datawriting). Here, charge for supply of a potential level or charge forsupply of a different potential level (hereinafter referred to aslow-level charge and high-level charge) is given. After that, thepotential of the fourth wiring is set to a potential at which thetransistor 162 is turned off, so that the transistor 162 is turned off.Thus, the charge given to the node FG is held (data holding).

Since the off-state current of the transistor 162 is extremely small,the charge of the gate electrode of the transistor 160 is held for along time.

Next, reading of data will be described. When an appropriate potential(reading potential) is supplied to the fifth wiring while apredetermined potential (fixed potential) is supplied to the firstwiring, the potential of the second wiring varies depending on theamount of charge held in the node FG. This is generally because when thetransistor 160 is an n-channel transistor, apparent threshold voltageV_(th) _(—) _(H) in the case where a high-level charge is supplied tothe node FG (also referred to as the gate electrode of the transistor160) is lower than apparent threshold voltage V_(th) _(—) _(L) in thecase where a low-level charge is supplied to the node FG. Here, theapparent threshold voltage refers to the potential of the fifth wiring,which is needed to turn on the transistor 160. Thus, the potential ofthe fifth wiring is set to a potential V₀ between V_(th) _(—) _(H) andV_(th) _(—) _(L), whereby charge supplied to the node FG can bedetermined. For example, in the case where a high-level charge issupplied in writing, when the potential of the fifth wiring is V₀(>V_(th) _(—) _(H)), the transistor 160 is turned on. In the case wherea low-level charge is supplied in writing, even when the potential ofthe fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 160 remainsoff. Therefore, the data held can be read by measuring the potential ofthe second wiring.

Note that in the case where memory cells are arrayed, only data ofdesired memory cells need to be read. In the case where such reading isnot performed, a potential at which the transistor 160 is turned offregardless of the state of the gate electrode of the transistor 160,that is, a potential smaller than V_(th) _(—) _(H) may be supplied tothe fifth wiring. Alternatively, a potential at which the transistor 160is turned on regardless of the state of the gate electrode, that is, apotential higher than V_(th) _(—) _(L) may be supplied to the fifthwiring.

When a transistor which includes a channel formation region formed usingan oxide semiconductor and has extremely small off-state current isapplied to the semiconductor device in this embodiment, thesemiconductor device can hold data for an extremely long period. Inother words, refresh operation is not needed or the frequency of therefresh operation can be extremely low, which results in a sufficientreduction in power consumption. Moreover, stored data can be held for along time even during a period in which power is not supplied (thepotential is preferably fixed).

Further, the semiconductor device described in this embodiment does notneed high voltage for writing data and has no problem of deteriorationof elements. For example, unlike a conventional non-volatile memory, itis not necessary to inject and extract electrons into and from afloating gate; thus, a problem such as deterioration of a gateinsulating film does not occur at all. In other words, the semiconductordevice according to one embodiment of the present invention does nothave a limit on the number of write cycles, which is a problem in aconventional nonvolatile memory, and reliability thereof is drasticallyimproved. Furthermore, data is written depending on the on state or theoff state of the transistor, whereby high-speed operation can be easilyachieved.

Further, in the transistor 162, the low-resistance region 144 a (or thelow-resistance region 144 b) in the oxide semiconductor film is incontact with the source electrode 142 a (or the drain electrode 142 b)to be electrically connected thereto, so that contact resistance can bereduced; thus, the transistor 162 can have excellent electriccharacteristics (e.g., high on-state current). Therefore, the use of thetransistor 162 allows higher performance of the semiconductor device.Moreover, the transistor 162 has high reliability; thus, higherreliability of the semiconductor device can be achieved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

In this embodiment, a semiconductor device which includes the transistordescribed in Embodiment 1, can hold stored data even when not powered,does not have a limitation on the number of write cycles, and has astructure different from the structure described in Embodiment 3 will bedescribed with reference to FIGS. 7A and 7B and FIGS. 8A and 8B. Notethat the transistor 162 included in the semiconductor device in thisembodiment is the transistor described in Embodiment 1. Any of thestructures of the transistors described in previously-cited Embodimentscan be employed for the transistor 162.

FIG. 7A illustrates an example of a circuit configuration of asemiconductor device, and FIG. 7B is a conceptual diagram illustratingan example of a semiconductor device. First, the semiconductor deviceillustrated in FIG. 7A will be described, and then the semiconductordevice illustrated in FIG. 7B will be described.

In the semiconductor device illustrated in FIG. 7A, a bit line BL iselectrically connected to one of the source electrode or the drainelectrode of the transistor 162, a word line WL is electricallyconnected to the gate electrode of the transistor 162, and the other ofthe source electrode or the drain electrode of the transistor 162 iselectrically connected to a first terminal of a capacitor 164.

Moreover, the transistor 162 including an oxide semiconductor hasextremely small off-state current. For that reason, the potential of thefirst terminal of the capacitor 164 (or charge accumulated in thecapacitor 164) can be held for an extremely long period by turning offthe transistor 162.

Next, writing and holding of data in the semiconductor device (a memorycell 250) illustrated in FIG. 7A will be described.

First, the potential of the word line WL is set to a potential at whichthe transistor 162 is turned on, so that the transistor 162 is turnedon. Thus, the potential of the bit line BL is supplied to the firstterminal of the capacitor 164 (data writing). After that, the potentialof the word line WL is set to a potential at which the transistor 162 isturned off, so that the transistor 162 is turned off. Thus, thepotential at the first terminal of the capacitor 164 is held (dataholding).

Since the off-state current of the transistor 162 is extremely small,the potential of the first terminal of the capacitor 164 (or the chargeaccumulated in the capacitor) can be held for a long time.

Next, reading of data will be described. When the transistor 162 isturned on, the bit line BL which is in a floating state and thecapacitor 164 are electrically connected to each other, and the chargeis redistributed between the bit line BL and the capacitor 164. As aresult, the potential of the bit line BL changes. The amount of changein the potential of the bit line BL varies depending on the potential ofthe first terminal of the capacitor 164 (or the charge accumulated inthe capacitor 164).

For example, the potential of the bit line BL after chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 164, C is the capacitance of thecapacitor 164, C_(B) is the capacitance of the bit line BL (hereinafteralso referred to as “bit line capacitance”), and V_(B0) is the potentialof the bit line BL before the charge redistribution. Therefore, it canbe found that assuming that the memory cell 250 is in either of twostates in which the potentials of the first terminal of the capacitor164 are V_(i) and V₀ (V₁>V₀), the potential of the bit line BL in thecase of holding the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) ishigher than the potential of the bit line BL in the case of holding thepotential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparison between the potential of the bit line BL and apredetermined potential, data can be read.

As described above, the semiconductor device illustrated in FIG. 7A canhold charge that is accumulated in the capacitor 164 for a long timebecause the off-state current of the transistor 162 is extremely small.In other words, refresh operation is not needed or the frequency ofrefresh operation can be extremely low, which results in a sufficientreduction in power consumption. Moreover, stored data can be held for along time even during a period in which power is not supplied.

Next, the semiconductor device illustrated in FIG. 7B will be described.

The semiconductor device illustrated in FIG. 7B includes a memory cellarray 251 (memory cell arrays 251 a and 251 b) including a plurality ofmemory cells 250 illustrated in FIG. 7A as memory circuits in the upperportion, and a peripheral circuit 253, which is necessary for operatingthe memory cell arrays 251 a and 251 b, in the lower portion. Note thatthe peripheral circuit 253 is electrically connected to the memory cellarray 251 a and the memory cell array 251 b.

In the structure illustrated in FIG. 7B, the peripheral circuit 253 canbe provided directly under the memory cell arrays 251 a and 251 b. Thus,a reduction in the size of the semiconductor device can be achieved.

It is preferable that a semiconductor material of the transistorprovided in the peripheral circuit 253 be different from that of thetransistor 162. For example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material can operate at sufficiently high speed.Thus, the transistor enables a variety of circuits (e.g., a logiccircuit and a driver circuit) which need to operate at high speed to befavorably obtained.

Note that FIG. 7B illustrates, as an example, the semiconductor devicein which two memory cell arrays, the memory cell array 251 a and thememory cell array 251 b, are stacked; however, the number of memory cellarrays to be stacked is not limited thereto. Three or more memory cellarrays may be stacked.

Next, a specific structure of the memory cell 250 illustrated in FIG. 7Awill be described with reference to FIGS. 8A and 8B.

FIGS. 8A and 8B illustrate an example of a structure of the memory cell250. FIG. 8A is a cross-sectional view of the memory cell 250, and FIG.8B is a plan view of the memory cell 250. Here, FIG. 8A corresponds to across section along line I-J and line K-L in FIG. 8B.

The transistor 162 in FIGS. 8A and 8B can have the same structure as thetransistor in Embodiment 1.

An interlayer insulating film 149 having a single-layer structure or astacked-layer structure is provided over the transistor 162. Inaddition, a conductive film 153 is provided in a region overlapping withthe source electrode 142 a of the transistor 162 with the interlayerinsulating film 149 and the insulating film 150 interposed therebetween,and the source electrode 142 a, the interlayer insulating film 149, theinsulating film 150, and the conductive film 153 form a capacitor 164.That is, the source electrode 142 a of the transistor 162 functions asone electrode of the capacitor 164, and the conductive film 153functions as the other electrode of the capacitor 164.

An insulating film 152 is provided over the transistor 162 and thecapacitor 164. Further, a wiring 156 a and a wiring 156 b for connectingthe memory cell 250 to an adjacent memory cell 250 are provided over theinsulating film 152. The wiring 156 a is electrically connected to thesource electrode 142 a through the electrode formed in an opening formedin the interlayer insulating film 149, the insulating film 150, and theinsulating film 152. The wiring 156 b is electrically connected to thedrain electrode 142 b through the electrode formed in an opening formedin the interlayer insulating film 149, and the insulating films 150 and152. The wirings 156 a and 156 b may be electrically connected to thesource electrode 142 a and the drain electrode 142 b through anotherconductive film provided in the opening. Note that the wirings 156 a and156 b correspond to the bit line BL in the circuit diagram of FIG. 7A.

In FIGS. 8A and 8B, the drain electrode 142 b of the transistor 162 canalso function as a source electrode of a transistor included in anadjacent memory cell. With such a planar layout, the area occupied bythe semiconductor device can be reduced; thus, higher integration can beachieved.

When the planar layout illustrated in FIG. 8B is employed, the areaoccupied by the semiconductor device can be reduced; thus, the degree ofintegration can be increased.

As described above, the plurality of memory cells formed in multiplelayers in the upper portion each include a transistor including an oxidesemiconductor. Since the off-state current of the transistor includingan oxide semiconductor is small, stored data can be held for a long timewith the use of the transistor. In other words, the frequency of refreshoperation can be extremely lowered, which results in a sufficientreduction in power consumption.

A semiconductor device having a novel feature can be obtained by beingprovided with both a peripheral circuit including the transistorincluding a material other than an oxide semiconductor (in other words,a transistor capable of operating at sufficiently high speed) and amemory circuit including the transistor including an oxide semiconductor(in a broader sense, a transistor with sufficiently small off-statecurrent). Further, with a structure in which the peripheral circuit andthe memory circuit are stacked, higher integration of the integration ofthe semiconductor device can be achieved.

As described above, a miniaturized and highly-integrated semiconductordevice having high electrical characteristics and a method formanufacturing the semiconductor device can be provided.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, examples of application of the semiconductor devicedescribed in any of the above embodiments to portable devices such as amobile phone, a smartphone, or an e-book reader will be described withreference to FIGS. 9A and 9B, FIG. 10, FIG. 11, and FIG. 12.

In portable electronic devices such as a mobile phone, a smart phone,and an e-book reader, an SRAM or a DRAM is used to store image datatemporarily. This is because response speed of a flash memory is low andthus a flash memory is not suitable for image processing. On the otherhand, an SRAM or a DRAM has the following characteristics when used fortemporary storage of image data.

In a normal SRAM, as illustrated in FIG. 9A, one memory cell includessix transistors, which are a transistor 801, a transistor 802, atransistor 803, a transistor 804, a transistor 805, and a transistor806, and they are driven by an X decoder 807 and a Y decoder 808. A pairof transistors 803 and 805 and a pair of the transistors 804 and 806each serve as an inverter, and high-speed driving can be performedtherewith. However, an SRAM has a disadvantage of large cell areabecause one memory cell includes six transistors. Provided that theminimum feature size of a design rule is F, the area of a memory cell inan SRAM is generally 100 F² to 150 F². Therefore, the price per bit ofan SRAM is the highest among a variety of memory devices.

On the other hand, as illustrated in FIG. 9B, a memory cell in a DRAMincludes a transistor 811 and a storage capacitor 812, and is driven byan X decoder 813 and a Y decoder 814. One cell includes one transistorand one capacitor and has a small area. The area of a memory cell in aDRAM is generally less than or equal to 10 F². Note that the DRAM needsto be refreshed periodically and consumes electric power even when arewriting operation is not performed.

However, the area of the memory cell of the semiconductor devicedescribed in the above embodiments is about 10 F² and frequentrefreshing is not needed. Therefore, the area of the memory cell can bereduced, which results in a reduction in power consumption.

FIG. 10 is a block diagram of a portable device. A portable deviceillustrated in FIG. 10 includes an RF circuit 901, an analog basebandcircuit 902, a digital baseband circuit 903, a battery 904, a powersupply circuit 905, an application processor 906, a flash memory 910, adisplay controller 911, a memory circuit 912, a display 913, a touchsensor 919, an audio circuit 917, a keyboard 918, and the like. Thedisplay 913 includes a display portion 914, a source driver 915, and agate driver 916. The application processor 906 includes a CPU 907, a DSP908, and an interface (IF) 909. In general, the memory circuit 912includes an SRAM or a DRAM; by employing any of the semiconductordevices described in the above embodiments for the memory circuit 912,writing and reading of data can be performed at high speed, data can beheld for a long time, and power consumption can be sufficiently reduced.

FIG. 11 illustrates an example in which any of the semiconductor devicesdescribed in the above embodiments is used for a memory circuit 950 in adisplay. The memory circuit 950 illustrated in FIG. 11 includes a memory952, a memory 953, a switch 954, a switch 955, and a memory controller951. Further, the memory circuit 950 is connected to a displaycontroller 956 which reads and controls image data input through asignal line (input image data) and data stored in the memories 952 and953 (stored image data), and is also connected to a display 957 whichdisplays an image based on a signal input from the display controller956.

First, image data (input image data A) is formed by an applicationprocessor (not illustrated). The input image data A is stored in thememory 952 though the switch 954. The image data (stored image data A)stored in the memory 952 is transmitted to the display 957 through theswitch 955 and the display controller 956, and is displayed on thedisplay 957.

In the case where the input image data A is not changed, the storedimage data A is read from the memory 952 through the switch 955 by thedisplay controller 956 normally at a frequency of approximately 30 Hz to60 Hz.

Next, for example, when a user performs an operation to rewrite a screen(i.e., when the input image data A is changed), the applicationprocessor produces new image data (input image data B). The input imagedata B is stored in the memory 953 through the switch 954. Also duringthis time, the stored image data A is regularly read from the memory 952through the switch 955. After the completion of storing the new imagedata (stored image data B) in the memory 953, from the next frame forthe display 957, the stored image data B starts to be read, istransmitted to the display 957 through the switch 955 and the displaycontroller 956, and is displayed on the display 957. This readingoperation continues until another next new image data is stored in thememory 952.

By alternately writing and reading image data to and from the memory 952and the memory 953 as described above, images are displayed on thedisplay 957. Note that the memory 952 and the memory 953 are notnecessarily separate memories and a single memory may be divided andused. By employing any of the semiconductor devices described in theabove embodiments for the memory 952 and the memory 953, data can bewritten and read at high speed and held for a long time, and powerconsumption can be sufficiently reduced.

FIG. 12 is a block diagram of an e-book reader. The e-book reader inFIG. 12 includes a battery 1001, a power supply circuit 1002, amicroprocessor 1003, a flash memory 1004, an audio circuit 1005, akeyboard 1006, a memory circuit 1007, a touch panel 1008, a display1009, and a display controller 1010.

Here, the semiconductor device described in any of the above embodimentscan be used for the memory circuit 1007 in FIG. 12. The memory circuit1007 has a function of temporarily holding the contents of a book. Forexample, when a user reads an e-book, the user may use a highlightingfunction of changing a display color, drawing an underline, using a boldfont, changing the type of letter, or the like so that the specificportion is in clear contrast to the other portions. In the case wheredata of the portion specified by the user is held for a long time, thedata may be copied to the flash memory 1004. Also in such a case, thesemiconductor device described in any of the above embodiments is used,whereby writing and reading of data can be performed at high speed, datacan be stored for a long time, and power consumption can be sufficientlyreduced.

As described above, the semiconductor device in any of the aboveembodiments is mounted on each of the portable devices described in thisembodiment. Thus, it is possible to obtain a portable device which iscapable of reading data at high speed, storing data for a long time, andreducing power consumption.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial no.2011-282450 filed with Japan Patent Office on Dec. 23, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor film over an insulating surface; a gate insulating filmover the oxide semiconductor film; a gate electrode provided over thegate insulating film and overlapping with the oxide semiconductor film;a first insulating film over the gate insulating film and the gateelectrode; a source electrode in contact with one end of the oxidesemiconductor film and one end of the first insulating film; a drainelectrode in contact with the other end of the oxide semiconductor filmand the other end of the first insulating film; and a second insulatingfilm over the source electrode and the drain electrode, wherein heightsof top surfaces of the source electrode and the drain electrode aresubstantially the same as heights of top surfaces of the firstinsulating film and the second insulating film, and wherein a channellength of the oxide semiconductor film is greater than or equal to 1 nmand less than or equal to 30 nm.
 2. The semiconductor device accordingto claim 1, wherein a distance between the gate electrode and a contactregion in which the oxide semiconductor film is in contact with thesource electrode is greater than or equal to 1 nm and less than or equalto 30 nm.
 3. The semiconductor device according to claim 1, wherein thesemiconductor device further comprises a transistor including asemiconductor material having a band gap which is different from a bandgap of the oxide semiconductor film.
 4. The semiconductor deviceaccording to claim 1, wherein the semiconductor device is a memory cell.5. A semiconductor device comprising: an oxide semiconductor filmprovided over an insulating surface and including a channel formationregion, and a first low-resistance region and a second low-resistanceregion with the channel formation region sandwiched therebetween; a gateinsulating film over the oxide semiconductor film; a gate electrodeprovided over the gate insulating film and overlapping with the channelformation region; a first insulating film over the gate insulating filmand the gate electrode; a source electrode in contact with a part of thefirst low-resistance region; a drain electrode in contact with a part ofthe second low-resistance region; and a second insulating film over thesource electrode and the drain electrode, wherein heights of topsurfaces of the source electrode and the drain electrode aresubstantially the same as heights of top surfaces of the firstinsulating film and the second insulating film, and wherein a channellength of the oxide semiconductor film is greater than or equal to 1 nmand less than or equal to 30 nm.
 6. The semiconductor device accordingto claim 5, wherein a distance between the gate electrode and a contactregion in which the oxide semiconductor film is in contact with thesource electrode is greater than or equal to 1 nm and less than or equalto 30 nm.
 7. The semiconductor device according to claim 5, wherein thesemiconductor device further comprises a transistor including asemiconductor material having a band gap which is different from a bandgap of the oxide semiconductor film.
 8. The semiconductor deviceaccording to claim 5, wherein the semiconductor device is a memory cell.9. A semiconductor device comprising: an oxide semiconductor film overan insulating surface; a first insulating film over the oxidesemiconductor film; a first electrode provided over the first insulatingfilm and overlapping with the oxide semiconductor film; a secondinsulating film over the first insulating film and the first electrode;a second electrode in contact with one end of the oxide semiconductorfilm and one end of the second insulating film; a third electrode incontact with the other end of the oxide semiconductor film and the otherend of the second insulating film; and a third insulating film over thesecond electrode and the third electrode, wherein heights of topsurfaces of the second electrode and the third electrode aresubstantially the same as heights of top surfaces of the secondinsulating film and the third insulating film, and wherein a channellength of the oxide semiconductor film is greater than or equal to 1 nmand less than or equal to 30 nm.
 10. The semiconductor device accordingto claim 9, wherein a distance between the first electrode and a contactregion in which the oxide semiconductor film is in contact with thesecond electrode is greater than or equal to 1 nm and less than or equalto 30 nm.
 11. The semiconductor device according to claim 9, wherein thesemiconductor device further comprises a transistor including asemiconductor material having a band gap which is different from a bandgap of the oxide semiconductor film.
 12. The semiconductor deviceaccording to claim 9, wherein the semiconductor device is a memory cell.